Hyperoxia is often used to enhance tissue oxygenation when patients are suffering from respiratory distress. Unfortunately, its therapeutic benefits are limited by oxidative cell injury and death to pulmonary cells. The decision to repair oxidative damage or initiate cell death is dictated by genes, such as the tumor suppressor p53, that control cell fate. The major determinant of p53- mediated cell survival is the cyclin-dependent kinase inhibitor p21. Consistent with p21 being an important pro-survival molecule, mice and cell lines lacking p21 quickly succumb to hyperoxia. While searching for mechanisms by which p21 protects against hyperoxia, p21 was discovered to prevent the loss of Bcl-XL in cells exposed to hyperoxia. Gain and loss of function studies in cell lines provided experimental proof that Bcl-XL, an anti-apoptotic member of the Bcl-2 gene family, is a relevant target of p21-mediated protection against hyperoxia. Preliminary studies indicate p21 also regulates expression of other anti-apoptotic members of the Bcl-2 family. In contrast, p21 does not alter expression of the pro-apoptotic proteins Bax or Bak. Based upon these observations, we propose to test the hypothesis that p21 protects against hyperoxia by regulating expression of anti- apoptotic members of the Bcl-2 family. Using genetically modified cell line and mouse models, Aim 1 will identify all anti-apoptotic members of the Bcl-2 family whose expression is regulated by p21 during hyperoxia, Aim 2 will determine how hyperoxia and p21 regulates their expression, and Aim 3 will determine whether they mediate the cytoprotective effects of p21. Successful completion of these studies will clarify how p21 promotes survival of cells damaged by hyperoxia and provide new insight into how cell fate decisions are made in the oxidized lung. Relevance of Research for Public Health. Because persistent oxidative stress is an underlying cause of asthma, chronic inflammation, ischemia/reperfusion injury, cancer, the aging process, and many other diseases, understanding how p21 controls cell growth and survival during hyperoxia could provide new opportunities for promoting public health.